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MBR Design.
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MBR CourseMembrane Fouling & Cleaning
October 16 & 17, 2012
Hamid Rabie
Effect of Fouling on Permeability
initial permeability
Irreversible fouling
Restored permeability
Reversible fouling
Permeability after time t
Time
Per
mea
bilit
y
Irreversible fouling is a relative terminology
Fouling Loss of productivity due to various factors (physical/chemical):
membrane compaction
adsorption
membrane deposits and solid build up
pore plugging and precipitation
gel compaction
concentration polarization (less important for MBR)
Fouling prevention is major design/operational issues
Fouling can be controlled by Membrane Modification (pore size, porosity, structure, nature)
Pre-treatment (screening, pre-oxidation, )
Process Considerations (flux, recovery, sludge age, )
Maintenance Cleaning (back pulse, regular chemical cleaning, relaxation, ...)
Hydraulic Improvements (tank design, air distribution, ...)
Structure of an Asymmetric Membrane
Fouling can occur on surface or inside membrane structure.
Membrane compaction
changes membrane structure
and reduces porosity resulting
in higher resistance to flow.
15
20
25
30
0 5 10 15 20Time (day)
Wat
er p
erm
eabi
lity
(gfd
/psi
)
With 15min initial permeation
Adsorption Capacity
Fibres were soaked in bioreactor and clean water permeability was measured at different times
Rate of Decline in Permeability
Without initial permeation
0.05 gfd/psi/day
With 15min initial permeation
0.24 gfd/psi/day
Adsorbed material may be removed by extensive water rinsing or chemical cleaning.
Cake Formation and Concentration Polarization
High molecular weight solutes
Membrane
JW
CP
sub-layer
CS
Cb
JW
Gel layer or cake
Bulk flow
Note: C s > Cb
1. Decrease in back diffusiondue to established sub-layer(concentration polarization)
2. New condition at membranemay exceed the limit (e.g., 0.02% for silica)
3. Increase in C P (due to Cs > Cb )
4. Cake layer resistance
At Steady State:Rate of solutes coming towards
membrane due to filtration =
Rate of migration of solutes away from membrane due to diffusion,
shear and back pulse
All components in feed can foul the membrane
In many systems, the main foulant exists in trace amount
Common Foulants:
Scaling: CaCO3, CaSO4, BaSO4
Metal Oxides: iron, aluminum
Inorganic Colloids: silt
Silica
Organics: oil, NOM
Biofilms: bacteria and EPS (protein, nucleic acid, lipopolysaccharides,
DNA, )
Foulant is a mixture of chemicals usually in a complex
matrix
What Fouls the Membrane?
Membrane Biofouling
Accumulation and attachment of microorganisms (e.g.., bacteria, fungi, microalgae)
Complete growth of biofilm: bacteria is hidden in EPS matrix. Note fibrous structure.
Biofilm is in multilayer of living and dead cells a nd associated EPS.
Biofouling is more severe on MF and UF since nutrie nts can pass through.
Biofouling may result in degradation of membrane an d joint glue.
Cleaning Fundamentals
1. Competitive adsorption
2. Solubilization Changing solubility (e.g., increasing T)
Emulsifying
Dispersing
3. Chemical Modification
Hydrolysis of fats and oils (e.g., at high pH)
Oxidation (e.g., organics)
Degradation of proteins
Chelating (e.g., divalent cations)
Reaction of metal oxides and acids
Cleaning Mechanism
1. Acids: HCl, HNO3, H3PO4, C3H4(OH)(COOH)3
2. Alkalis: NaOH
3. Surfactants:anionic (sulfonates, sulfates, phosphates), cationic (quaternary ammonium
salts), nonionic (PEO: -O-CH2), competitive adsorption, emulsifying, micelle
4. Chelators: ethylenediamine tetraacetic acid (EDTA)
5. Oxidizers: NaOCl, ClO2, HOBr, H2O2, ozone, UV
6. Enzymes:degrade proteins, starch, fat, oil, cellulose, cleave peptide linkage in proteins
in specific sites, it is very selective and bacteria can adopt, very slow reaction
Common Cleaners
CaCO3 + 2HCl CaCl2 + H2O + CO2
Fe2O3 + 6HCl 2FeCl3 + 3H2O
Acid Cleaning
HCl cheap, high rate of reaction and yield, corrosive
H2SO4 cheap, moderate rate of reaction and yield, corrosi ve
HNO3 also oxidant for organics and biological, moderaterate of reaction and yield, too corrosive
H3PO4 chelating, pH buffering, less corrosive, uses too m uchfor pH < 2.3, Ca phosphates has limited solubility, tooexpensive, not recommended for P removal processes
Citric acid good chelating with Ca, its complex with ferrous io nhas low solubility, low kinetic, high yield
1. Dissolves: silicasome inorganic colloids (dispersion)many biological and organic foulant
2. Sanitizer
3. Neutralizes fatty acids and humic acids (R-COOH)
4. Hydrolysis fats and oils
C3H5-(OOCR)3 + 3NaOH C3H5(OH)3 + 3NaOOCRester soap
NaOOCR has emulsification properties
NaOOCR is insoluble at low pH, also precipitate with Ca, Mg, add in
presence of chelating agent (EDTA) to remove Ca, Mg
5. Hard to maintain pH, measure pH during cleaning
Alkalis Cleaning (NaOH)
1. Extremely effective specially for pore fouling
2. Disinfection property (biofouling control)
damage to cell wall
alteration of cell permeability
inhibition of enzyme activity
3. Alkali cleaning property (increases pH)
NaOCl + H2O HOCl + NaOHactive component
widely used and inexpensive
more effective at low pH and also more corrosive
harmful by-products (e.g., THM)
NaOCl (Oxidizer/Disinfectant)
Depends greatly on types of foulant and membrane and the
way membrane has been fouled (see examples)
Cleaning Sequence/Condition
1. Biofilm (dominant) + Inorganic
Alkalis/Oxidizer/Acid (initial acid cleaning increases adhesion of humic material, initial caustic cleaning can remove significant org anic so oxidizer cleaning becomes more efficient.)
2. Inorganic (dominant) + Organic + Biofilm
Mixture of Acids and Chelators/Oxidizer or Alkalis
Conditions:Type of Cleaner, Temperature, Concentration, Contac t TimeCertain relations exists, for example: bacteria kil l is related directly to dosage
The best sequence and condition can be obtained aft eridentifying foulant and preliminary tests.
Operational Considerations of Cleaning
Mechanical Cleaning
1. Scrubbing: solid removal
2. Back Pulse: reverse TMP and drive permeate backward
3. Relaxation: eliminate TMP, allow gel layer to dissipate
Chemical Cleaning
1. In-Situ/Empty Tank: back wash regularly with chemicals in CIP from top only,in pulses at moderate flow for uniform distribution
2. In-Situ/Full Tank: the same as empty tank except back wash from both ends
3. Soak: soak in chemical solution for extended period
Cleaning Methods
Backwash
Clean-In-Place Tank(Filtrate from membrane)
Process Tank Water
Backwash Cleaning(Reverse Flow with Filtrate)
Use clean filtrate to backwash membranes. A reverse flow from the CIPtank is fed to the inside of the membrane fibers cleaning from the inside out
Cleaning chemicals are optional and not always necessary.
X-section
Maintenance cleaning
Used for maintaining permeability (conditioning rate of decline)frequent, short contact time such as: back pulse, relaxation, in-situ/full tank
Recovery cleaning
Used for restoring permeability close to one of new membrane, not frequent, long contact time, such as: soak.
Cleaning Methods (Another Classification)
Flux Distribution in Fibres during Back Wash
0
0.4
0.8
1.2
1.6
0 0.5 1 1.5Fiber Length from top (m)
Nor
mal
ized
Flu
x
30 gfd
15 gfd
Empty Tank Back Wash(only from top)
0
0.4
0.8
1.2
1.6
0 0.5 1 1.5Fiber Length from top (m)
Nor
mal
ized
Flu
x
30 gfd
15 gfd
Full Tank Back Wash(from both sides)
Calculation is done based on a true permeability of 10 gfd/psi
Why using pulsed back wash athigher flow rates ?
1. uniform distribution
2. low chemical consumption
Calculation is done based on a true permeability of 10 gfd/psi
Long pipe lines reduces back wash performance
1. Membrane Limitations
pH
Temperature
Chemicals
2. Process Limitations
Bioreactor (e.g., nitrifiers sensitive to chlorine and pH)
Drinking Water (e.g., THM, residual chlorine)
Any Cleaning Should not Exceed:
Steps Towards Identifying Problem(s)
1. Solid build up
2. Aerator Status
3. Fibre appearance (black, brown, damaged, )
4. Fluctuations in designed operating conditions (flux, pH, )
Operating at, for example, high solid or flux can foul membranes in short time but it takes long before membranes recover.
5. Record any changes in operation, cleaning results, and any
suspected chemicals.
Check List
Fully characterise the feed
Cleaning of fouled fibres and analysis of extracts
Extraction of foulant and analysis (GS/MS, Pyrolysis, ...)
Autopsy of fouled membrane
ESEM (environmental scanning electron microscopy) for morphology
TEM (transmission electron microscopy) for biological fouling
EDX (energy dispersive x-ray) for elemental analysis
MWCO (molecular weight cut-off)
Pore size
Identifying Nature of Foulants
Membrane fouling can be controlled
Essential to diagnose what is the cause
Crucial to understand fluid dynamics and mass transfer
Crucial to understand chemical interactions with membrane
Develop rational pretreatment and cleaning protocols
Conclusions
Membrane Biofouling (STEM micro -graph)
1 micron
Note significant biofouling on the surface of membr ane
STEM micro -graph after Maintenance Cleaning
Note that the fouling has been controlled
1 micron
Fouled Membrane in Tap Water
Fouling is mainly due to fibrous organic matter; moderate microbial and
inorganic fouling.
STEM micro-graph of foulant
0.5 micron
Note the disperse/loose structure of foulant
Burlington tap water
Dead-end filtration
MBR Pilot with Ferric Addition
0.5 micron
ESEM Micro-graph
of fouled membrane
(Foulant Analysis)
EDX examination of foulant on membranes used in an MBR pilot
Foulant is mainly inorganic:iron and calcium precipitate
(almost 10 to 1 ratio)
Iron precipitates in different forms which have different optimum pH
values for precipitation
(Foulant Structure)
ProblemsStructure of inorganic fouling. Membrane pores were plugged with precipitates. Further analysis also showed bacteria growth typical of sewage treatment plants.
Conventional cleanings (chlorine or acid) did not recover permeability.
Solutions
A mixture of chelating agents and acid for cleaning
Regular maintenance cleaning with formulated chemical according to the Manual
Control of inorganic fouling: pH adjustment, flash mixer, low dosage
ESEM analysis of foulant at an MBR pilot